(1) Field of the Invention
This invention relates to methods and apparatus for applying a coating solution such as photoresist, SOG (Spin On Glass, also called a silica coating material) or polyimide resin to substrates such as semiconductor wafers, glass substrates for photomasks, glass substrates for liquid crystal displays or glass substrates for optical disks (hereinafter referred to simply as substrates) to form a film of the coating solution on the surface of each substrate. More particularly, the invention relates to a technique of supplying a coating solution to a substrate spun at a predetermined low rotational frequency, and thereafter spinning the substrate at a predetermined high rotational frequency to form a film of the coating solution in a desired thickness.
(2) Description of the Related Art
A conventional coating solution applying method of the type noted above will be described, based on the apparatus shown in FIG. 1.
This figure shows a principal portion of a substrate spin coating apparatus. The apparatus includes a suction type spin chuck 10 for suction-supporting and spinning a substrate or wafer W in a substantially horizontal posture, and a coating solution supply nozzle 30 disposed substantially over the center of spin for supplying a photoresist solution R, which is a coating solution, to a surface of wafer W.
The apparatus with this construction controls rotational frequency as shown in the time chart of FIG. 2, to form a photoresist film in a desired thickness on the surface of wafer W.
First, the spin chuck 10 is driven by a motor, not shown, to spin the wafer W at a predetermined low rotational frequency R1 (e.g. 900 rpm). At a point of time at which the spin stabilizes, photoresist solution R begins to be delivered at a substantially constant flow rate from the supply nozzle 30 (referenced t.sub.S in FIG. 2). Photoresist solution R continues to be supplied to a region around the spin center of wafer W. The supply of photoresist solution R is stopped at a point of time (t.sub.E in FIG. 2) which is a predetermined time after the photoresist supply starting point t.sub.S. Then, the rotational frequency of the spin chuck 10 is increased from rotational frequency R1 to rotational frequency R2 (e.g. 3,000 rpm). This higher rotational frequency R2 is maintained for a predetermined time. Consequently, a superfluous part of photoresist solution R supplied to the surface of wafer W is dispelled, thereby forming a photoresist film in a desired thickness on the surface of wafer W.
In the conventional method described above, a photoresist film is formed as a result of a behavior of photoresist solution R as schematically shown in FIGS. 3A through 3F. In these figures, wafer W is shown in circles and photoresist solution R in hatched regions, for simplicity of illustration. The varied rotational frequencies of wafer W are schematically indicated by different sizes of arrows in the figures.
Immediately after commencement of photoresist supply to the surface of wafer W spinning at the slow, supplying rotational frequency R1, as shown in FIG. 3A, photoresist solution R is present around the spin center of wafer W in the form of a drop Ra circular in plan view (which is hereinafter referred to as core Ra). As photoresist solution R continues to be supplied, the centrifugal force generated by the spin spreads the core Ra concentrically toward the edge of wafer W while allowing the core Ra substantially to retain the circular shape.
The core Ra retains the circular shape for a while (e.g. for several seconds), and thereafter undergoes conspicuous changes in shape. Specifically, as seen in FIG. 3A, photoresist solution R begins to flow in a plurality of rivulets (hereinafter referred to as fingers Rb) extending radially from the edge of circular core Ra toward the edge of wafer W. These numerous fingers Rb, by the centrifugal force, continue to grow toward the edge of wafer W with an increase in the diameter of core Ra as shown in FIG. 3B. The fingers Rb have a larger turning radius, and are therefore subjected to a greater centrifugal force, than the core Ra. Consequently, the fingers Rb grow toward the edge of wafer W faster than the enlargement of core Ra.
As photoresist solution R continues to be supplied to the wafer W spinning at the same supplying rotational frequency R1, leading ends of fingers Rb reach the edge of wafer W as shown in FIG. 3C. With the fingers Rb having reached the edge of wafer W, the photoresist solution R flows from the core Ra through the fingers Rb to the edge of wafer W to be scattered away (in scattering photoresist solution Rc). As the diameter of the core Ra increases further, the fingers Rb become broader as shown in two-dot-and-dash lines in FIG. 3C and FIG. 3D. As a result, regions between the fingers Rb not covered by photoresist solution R gradually diminish, until finally the entire surface of wafer W is covered by photoresist solution R (core Ra and fingers Rb) (FIG. 3E). Timing is determined beforehand for stopping the delivery of photoresist solution R through the supply nozzle 30 at this point of time (reference t.sub.E in FIG. 2).
After the entire surface of wafer W is covered with photoresist solution R as above, the rotational frequency of wafer W is increased from the supplying rotational frequency R1 to the faster, film-forming rotational frequency R2. A superfluous part of photoresist solution R covering the surface of wafer W is dispelled (as excess photoresist solution Rd), thereby forming a photoresist film R' in a desired thickness on the surface of wafer W (FIG. 3F).
The conventional method described above has the following drawback.
When the numerous fingers Rb reach the edge of wafer W, as shown in FIG. 3C, a large part of photoresist solution R subsequently supplied becomes scattering photoresist solution Rc flowing from the core Ra through the fingers Rb to be cast off to the ambient environment. A large quantity of photoresist solution R must therefore be supplied before the entire surface of wafer W is covered by the photoresist solution R, leading to an excessive consumption of photoresist solution R.
That is, the photoresist film of desired thickness is obtained with a very low efficiency of using photoresist solution R. A coating solution such as photoresist solution is far more expensive than a treating solution such as a developer or a rinse. Thus, a reduction in the quantity of unused, scattering coating solution is an important consideration in achieving low manufacturing costs of semiconductor devices and the like.